A reader type that can be checked for EOF without read.

main.go

package main

import (
	"io"
	"io/ioutil"
	"log"
	"os"
	"sync/atomic"
)

// eofReader can be checked for EOF, without a Read. It keeps track of the
// number of bytes read and compares it to the file size, in case the
// underlying type is a file.
type eofReader struct {
	r     io.Reader
	count uint64
}

// AtEOF returns true, if the number of bytes read equals the file size. False
// otherwise.
func (r *eofReader) AtEOF() (bool, error) {
	f, ok := r.r.(*os.File)
	if !ok {
		return false, nil
	}
	fi, err := f.Stat()
	if err != nil {
		return false, err
	}
	return r.Count() == uint64(fi.Size()), nil
}

// Read reads and counts.
func (r *eofReader) Read(buf []byte) (int, error) {
	n, err := r.r.Read(buf)
	atomic.AddUint64(&r.count, uint64(n))
	return n, err
}

// Count returns the count.
func (r *eofReader) Count() uint64 {
	return atomic.LoadUint64(&r.count)
}

func main() {
	f, err := os.Open("main.go")
	if err != nil {
		log.Fatal(err)
	}

	r := &eofReader{r: f}
	log.Println(r.AtEOF())

	if _, err = ioutil.ReadAll(r); err != nil {
		log.Fatal(err)
	}

	log.Println(r.AtEOF())
}

// 2016/12/19 03:49:35 false <nil>
// 2016/12/19 03:49:35 true <nil>